Slags

Slags, sometimes called Scorize, are fused compounds of silica with lime, alumina, and other substances. Blast-furnace slag is usually little else than a silicate of lime and alumina. In smelting processes the slag floats on the top of the molten metal, and is run off or raked off as 'waste material,' provided that the metal has been practically all extracted from it. Many slags which were thrown away in early times have been profitably smelted again in modern days, owing to the amount of metal left in them. Some slags form an opaque glass, but other varieties are more stone-like in appearance, while some are beautifully crystallised. In Great Britain about 18,000,000 tons of iron blast-furnace slag are annually produced. Until comparatively recent times this was considered useless material, but it is now utilised in several ways. By the action of steam upon it in the melted state it is made into fine threads or filaments called 'slag wool' or 'silicate cotton.' This is a bad conductor of heat and sound, and is used as a covering to boilers, and to prevent sound passing through floors. It is also employed for fireproof netting. In some cases blast-furnace slag has been made into serviceable bricks and large blocks for building, as well as into paving setts. On the Continent a useful building cement, or substitute for mortar, has been made from it. The slag from the manufacture of steel from Cleveland pig-iron by the basic process contains about 17 per cent. of phosphoric acid, and forms a valuable fertiliser for sour, peaty, and clay soils.